Concrete is the most consumed man-made material in the world. A typical concrete is made by mixing Portland cement, water and aggregates such as sand and crushed stone. Portland cement is a synthetic material made by burning a mixture of ground limestone and clay, or materials of similar composition in a rotary kiln at a sintering temperature of around 1,450° C.
Pavers are concrete blocks that are made by using a casting process, a pressing process, a compacting process, or a combination of vibration and pressing. Pavers are generally laid in interlocking pattern. These pavers are also sometime referred as paving stones. These pavers can be removed when damaged during service life with a new one reducing any service interruption. Interlocking pavers could be designed to have a gap between the patterns that provides for draining of water to sub layers.
ASTM C 936 provides criteria that concrete pavers need to satisfy but is not limited to the following: an average compressive strength of 8,000 psi; an average water absorption no greater than 5%; and resistance to at least 50 freeze-thaw cycles with average material loss not exceeding 1%. In addition to the ASTM requirements, one may also wish that the pavers satisfy additional requirements, including reduced efflorescence (e.g., reduced leaching out of reaction product due to concentration gradients); good color retention; and abrasion resistance depending on where the pavers are being used.
Like pavers, blocks are also pre-cast concrete produced either by casting or pressing processes, or similar compacting processes. Blocks are also referred to as concrete masonry units (CMUs), hollow blocks and concrete blocks. When these blocks are made with fly ash they are called cinder blocks. These blocks generally have a hollow structure. Artificial or man-made paving stones and construction block materials have been studied in efforts to replace the expensive and scarce natural material with low-cost, readily produced mimics. Such efforts, however, have yet to produce in a synthetic material that possesses the desired appearance, texture, density, hardness, porosity and other aesthetics characteristic of stone while at the same can be manufactured in large quantities at low cost with minimal environmental impact.
Blocks are expected to provide better structural property compared to clay bricks (for load bearing masonry structure), and a smoother surface when producing a masonry wall. In addition, interlocking concrete masonry units do not require mortar to bind the units. Some blocks can be used to build a hollow structure that results in good sound and thermal insulation as compared to a solid structure.
Blocks have to generally comply with the requirements of ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units. Blocks that comply with this standard are ensured to be acceptable as regards strength, geometry, durability and fire resistance, and are generally acceptable for use in standard commercial construction projects.
Hollow-core slabs, sometimes referred to as voided slabs or hollow core planks, are precast slabs of concrete. They are often used in building constructions, for example, as floors, walls or roofs in multi-story buildings. The precast concrete slab typically has tubular voids extending the full length of the slab, making the slab lighter than a massive floor slab of equal thickness or strength. Reduced weight lowers material and transportation cost.
Typical slabs are about 120 cm wide with a standard thickness between 15 cm and 50 cm. The precast concrete I-beams between the holes contain steel wire ropes that provide bending resistance to bending moment from loads. The manufacturing process involves extruding wet concrete around the prestressed steel wire rope from a moving mold. After curing the continuous slab is cut according to the required lengths and width. Hollow-core floor slabs are also made in rebar reinforced concrete (not prestressed). Hollow-core wall panels are made without reinforcement.
Concrete products, however, are not optimal in terms of both economics and environmental impact. Existing production technologies involve large energy consumption and carbon dioxide emission, leading to unfavorable carbon footprints. Portland cement manufacturing is not only an energy-intensive process, but also one that releases considerable quantities of greenhouse gas (CO2). The cement industry accounts for approximately 5% of global anthropogenic CO2 emissions. More than 60% of such CO2 comes from the chemical decomposition or calcination of limestone.
Recently, a revolutionary form of cement that is based on carbonatable calcium silicate materials has emerged as a promising substitute to traditional cements. Production of carbonatable calcium silicate-based cements involves significantly reduced CO2 emissions and energy consumption. In addition, this new cement sequesters CO2 when cured into concrete products because CO2 is needed to react with the carbonatable calcium silicate materials during the curing process to form concrete products.
Thus, there is an on-going need for novel and improved cement and concrete products and production technologies that can be mass applied at lower cost with improved energy consumption and more desirable carbon footprint.